1. Field of the Invention
This invention relates to multi-hole film cooled combustor liners for use in gas turbine engines, and more particularly, combustor liners having rectangular cooling film restarting holes to restart cooling films that are swallowed by large film interrupting apertures.
2. Description of Related Art
Combustor liners are generally used in the combustion section of a gas turbine engine which is located between the compressor and turbine sections of the engine. Combustor liners are also used in the exhaust section of aircraft engines that have afterburners. Combustors generally include an exterior casing and an interior combustor wherein fuel is burned producing a hot gas usually at an intensely high temperature such as 3,000.degree. F. or even higher. To prevent this intense heat from damaging the combustor casing before it exits to a turbine, a heat shield or combustor liner is provided in the interior of the combustor. This combustor liner thus prevents the intense combustion heat from damaging the combustor casing and the rest of the engine.
Many designs have been developed to cool the liners with cooling air taken from the cooling air passageway between the liner and casings. Prior methods for film cooling combustion liners provided circumferentially disposed rows of film cooling slots such as those depicted in U.S. Pat. No. 4,566,280 by Burr and U.S. Pat. No. 4,733,538 by Vdoviak et al. which are typified by complex structures that have non-uniform liner thicknesses which give rise to thermal gradients which cause low cycle fatigue in the liner and therefore shorten their potential life expectancy and reduce their durability. The complex shapes and machining required to produce these liners negatively effects their cost and weight.
A more detailed discussion of the related art may be found in a related U.S. Pat. No. 5,181,379 , entitled "GAS TURBINE ENGINE MULTI-HOLE FILM COOLED COMBUSTOR LINER AND METHOD OF MANUFACTURE", invented by Wakeman et al., issued Jan. 26, 1993, assigned to the same assignee, and incorporated herein by reference. Wakeman et al. developed a single wall multi-hole film cooled liner wherein the liner was disposed directly in the cooling air passage between it and the casing thereby forming the inner boundary of the cooling air passage.
Multi-hole film cooling holes are small, spaced closely together, sharply angled downstream with respect to flow along the liner and preferably cylindrical so as to form at least one continuous pattern designed to inject cooling air from the cold side of the liner onto the hot side in order to provide film cooling over the length of the liner. One particular design of a multi-hole film cooled annular liner clocks the multi-hole film cooling holes with a circumferential angle with respect to the combustor centerline of between 30 and 65 degrees. This design is the subject of a U.S. Pat. Application No. 07/614,368, and is entitled "COMBUSTOR LINER WITH CIRCUMFERENTIALLY ANGLED FILM COOLING HOLES" by Phillip D. Napoli, filed Nov. 15, 1990, assigned to the same assignee, and incorporated herein by reference.
Another particular feature of a multi-hole film cooled annular liner provides a liner that is arched and that is particularly useful for outer annular liners for which it provides buckling resistance. This design is the subject of a U.S. Pat. Application No. 07/591,311, and is entitled "ARCHED COMBUSTOR" by Hubert S. Roberts et al., assigned to the same assignee, and also incorporated herein by reference. These references provide background information for the present invention and are incorporated herein by reference.
The flow of air from the cooling air passage through the multi-hole film cooling holes and the dilution holes is possible because combustors are designed to create a positive pressure differential across the liner from the cooling air passage to the combustion zone. Pressure differentials significantly larger than prior art effusion cooled combustor liners are needed for multi-hole film cooling to inject cooling air onto the hot side of the liner through the small multi-hole film cooling holes and with sufficient velocity to form the cooling film. The arched combustor liner, as dislosed in the "ARCHED COMBUSTOR" patent above, provides significant bucking resistance with a minimum effect on the aerodynamics of the combustor and the complexity and cost of the combustor liner which is an expensive replacement part.
Combustor liners conventionally have an additional feature referred to as dilution holes which are much larger than the multi-hole film cooling holes and are designed to pass air into the combustion zone within the combustor liner to dilute the hot gases in the combustion zone and sometimes to further combust the fuel mixture in the combustion zone. Dilution holes, in a multi-hole film cooled liner, swallow the cooling film producing a cooling film dry or unwetted area just downstream, with respect to the film cooling air direction, of the dilution holes. These dry areas extend downstream till the multi-hole film cooling holes have reestablished the cooling film and to such an extent that significant portions of the liner just downstream of the dilution holes are subject to substantial more thermal degradation than the cooling film wetted portions of the liner. This causes hot spots in these dry areas due to the inability of the cooling film to restart for a significant length just downstream of the dilution holes. A sort of cooling film shadow is cast by the dilution holes with respect to cooling film flow on the liner. Circumferential spacing of the dilution holes cause circumferential thermal variations and their attendant thermal differential stresses.
Therefore in order to incorporate the advantages of low weight multi-hole film cooled combustor liners capable of withstanding the temperatures and pressure differentials found in combustors the inventor has developed a means to avoid a discontinuity in the cooling film caused by dilution holes in the cooling film shadow area caused by the dilution holes.